1. Field of the Invention
This invention relates to a process of gasifying solid fuels with gasifying agents containing oxygen, water vapor and, if desired, carbon dioxide, in a moving bed and in a fluidized bed. Coarse-grained solid fuels are gasified under a pressure of 5 to 150 bars in a moving bed, which is slowly descending. The gasifying agents are introduced into said bed from below. Incombustible mineral constituents are withdrawn from the bottom of the bed as solid ash or liquid slag. A product gas at temperatures of 25.degree. to 700.degree. C. is withdrawn from the moving bed gasifier and is cooled to temperatures of 120.degree. C. to form an aqueous condensate (gas liquor), which contain distillation products. Fine-grained solid fuels are gasified in the fluidized bed at lower pressures than are used in the moving bed and product gas at a temperature of 800.degree. to 1200.degree. C. is thus produced.
2. Discussion of Prior Art
It is know that solid fuels, such as coal, lignite or peat, can be gasified in a moving bed, which is sometimes ca--ed a fixed bed or in a fluidized bed. The gasification in a moving bed is described, e.g., in Ullmann's Encyklopadie der technischen Chemie, 4th edition (1977), on pages 383 to 386 of volume 14. Details of that known gasification process are apparent from U.S. Pat. Nos. 3,540,867 and 3,854,895. The gasification of solid fuels in a moving bed with a withdrawal of liquid slag has been described in British Pat. Nos. 1,507,905; 1,506,671 and 1,512,677. U.S. Pat. No. 4,347,064 and German Offenlegungsschrift No. 31 13 993 disclose the gasification in a circulating fluidized bed. A process in which a gasification in a moving bed and a gasification in a fluidized bed are combined is described in German Offenlegungsschrift No. 31 38 124 corresponding to U.S. Pat. No. 4,426,810 of January 24, 1984.
A granular fuel to be gasified in a moving bed should preferably have particle sizes in the range from 3 to 80 mm so that the bed is sufficiently permeable to gas. A moving bed can be used to gasify solid fuels at a high throughput rate and with a high efficiency but the raw gas is rich in distillation products because a countercurrent operation is performed in which the raw gas leaving the fuel bed is at a relatively low temperature, in most cases in the range from about 300.degree. to 650.degree. C. At these temperatures the distillation products escaping from the upper portion of the fuel bed, such as tar, oil, naphtha, phenols and fatty acids, are not decomposed but remain in the raw gas leaving the gas generator. When the raw gas is subsequently cooled, a major part of these distillation products condenses together with the water vapor to form a condensate known as gas liquor.
The gas liquor must be withdrawn from the process and must be separated into its components. This is effected in known manner, e.g., in plants for extracting the phenols, for stripping off the ammonia, and for removing residual phenols and fatty acids by a biological treatment.